Optical modulation apparatus and method of controlling optical modulator

ABSTRACT

An optical modulator having a voltage-optical output characteristic in which optical output varies periodically with respect to a voltage value of an electrical drive signal is driven by a modulator driving voltage signal, which has an amplitude of 2·Vπ between two light-emission culminations or two light extinction culminations of the voltage-optical output characteristic. A low-frequency superimposing unit superimposes a prescribed low-frequency signal on the modulator driving voltage signal, and an operating-point controller controls the operating point of the optical modulator by detecting operating-point drift of the optical modulator based upon the low-frequency signal component contained in an optical signal output from the optical modulator and controlling the bias voltage of the optical modulator in dependence upon the drift of the operating point of the optical modulator.

This application is a divisional application of application Ser. No.09/348,166, filed on Jul. 7, 1999, now U.S. Pat. No. 6,278,539.

BACKGROUND OF THE INVENTION

This invention relates to an optical modulation apparatus and to amethod of controlling an optical modulator. More particularly, theinvention relates to an optical modulation apparatus and to a method ofcontrolling an optical modulator, wherein even if the operating point ofan optical modulator the optical output of which varies periodicallywith respect to a driving voltage fluctuates owing to a change inambient temperature or aging, the fluctuation in operating point can becompensated for in stable fashion. More specifically, the presentinvention relates to a control method for stabilizing the operatingpoint of a Mach-Zehnder optical modulator (referred to as an “MZ-typeoptical modulator) in an optical transmitter used in a time-divisionmultiplexing (TDM) or wavelength-division multiplexing (WDM) opticaltransmission system.

The explosive increase in the quantity of available information inrecent years has made it desirable to enlarge the capacity and lengthenthe distance of optical communications systems. In-line opticalamplifier systems which accommodate a transmission speed of 10 Gbps arenow being put to practical use. Even greater capacity will be requiredin the future, and research and development is proceeding in both theTDM and WDM aspects of optical transmission.

Direct modulation

Intensity modulation and direct detection (so-called “directmodulation”) is the simplest technique to use for an electro-opticconversion circuit employed in an optical communications system.According to this technique, a current that activates a semiconductorlaser is turned on and off directly by the “0”s and “1”s of a datasignal to control the emission and extinction of the laser beam. When alaser per se is turned on and off directly, however, the light signalexperiences a fluctuation in wavelength (so-called “chirping”) owing tothe properties of the semiconductor. The higher the transmission speed(bit rate) of the data, the greater the influence of chirping. Thereason for this is that an optical fiber exhibits a chromatic dispersionproperty wherein propagation velocity varies for different wavelengths.When chirping is caused by direct modulation, propagation velocityfluctuates, waveforms are distorted during propagation through opticalfiber and it becomes difficult to perform long-distance transmission andtransmission at high speed.

External modulation

For the reasons mentioned above, external modulation is used for hightransmission speeds of 2.5 to 10 Gbps. According to external modulation,a laser diode emits light continuously and the emitted light is turnedon and off by the “1”s and “0”s of data using an external modulator. Theabove-mentioned MZ-type modulator primarily is used as the externalmodulator. FIGS. 32A and 32B are diagrams useful in describing theMZ-type modulator, in which FIG. 32A is a schematic view of theconstruction of the modulator and FIG. 32B is for describing themodulating operation.

Shown in FIG. 32A are a distributed-feedback semiconductor laser diode(DFB-LD) 1 used in long-distance transmission at a speed of greater than1 Gbps, an MZ-type modulator 2 and optical fibers 3 a, 3 b. The MZ-typemodulator 2 includes on an LiNbO₃ substrate, (1) an input opticalwaveguide 2 a formed on the substrate for introducing light from thelaser diode 1, (2) branching optical waveguides 2 b, 2 c and (3) anoutput optical waveguide 2 d formed on the substrate for outputtingmodulated light; (4) two signal electrodes 2 e, 2 f formed on thesubstrate for applying phase modulation to the optical signals in thebranching optical waveguides 2 b, 2 c, and (5) a signal input terminal 2g formed on the substrate for inputting an NRZ electrical drive signalto one of the signal electrodes, namely the electrode 2 e.

If a voltage applied to the signal electrodes 2 e, 2 f is controlled bythe “1”s and “0”s of data, the branching optical waveguides 2 b, 2 cdevelop a difference in refractive index and the light waves of theoptical signals in the optical waveguides develop a difference in phasebetween them. For example, if the data is a “0”, control is performed insuch a manner that the phase difference between the light waves of theoptical signals in the two optical waveguides 2 b, 2 c becomes 180°; ifthe data is a “1”, control is performed in such a manner that the phasedifference between the light waves of the optical signals in the twooptical waveguides 2 b, 2 c becomes 0°. If this arrangement is adopted,superimposing the optical signals of the two optical waveguides 2 b, 2 cwill make it possible to output the input light upon modulating it(turning it on and off) by the “1”s and “0”s of the data.

As shown in FIG. 32B, the optical output characteristic of the MZ-typeoptical modulator, which has a voltage difference between the twoelectrodes thereof, varies periodically in dependence upon the appliedvoltage. Point A represents the culmination of the light emission andpoint B the culmination of extinction. The range of the voltage over oneperiod is 2Vπ. When data is a “1”, therefore, voltage having anamplitude of Vπ is applied between the signal electrodes 2 e, 2 f,whereby light is emitted. When data is a “0”, a voltage of zero isapplied between the signal electrodes 2 e, 2 f, whereby light isextinguished.

The MZ-type optical modulator described above is advantageous in thattransmitted light exhibits little chirping. However, a change in thetemperature of the LiNbO₃ constituting the substrate, prolongedapplication of an electric field thereto and aging thereof areaccompanied by polarization of the substrate per se, electric chargeremains on the surface of the substrate and the bias voltage across thesignal electrodes fluctuates. Consequently, the voltage-optical outputcharacteristic of the MZ-type optical modulator fluctuates to the leftand right from the ideal curve a in FIG. 33 to the curves b and c. Inother words, the operating point of the MZ-type optical modulator driftswith the passage of time, thereby the on/off light level changes andcauses inter symbol interference between codes (refer to output eyepatter in FIG. 33).

Bias control method in NRZ modulation

Accordingly, in order to stabilize the operating point, the conventionalpractice is to perform control in such a manner that the bias voltage isincreased correspondingly if the curve shifts to the right and decreasedcorrespondingly if the curve shifts to the left. More specifically,there has been proposed a compensation method (referred to as “automaticbias-voltage control” (ABC) below) which includes superimposing alow-frequency signal on an electrical drive signal, detecting the amountof drift of the operating point and the direction of this drift, andcontrolling the bias voltage by feedback (see the specification ofJapanese Patent Application Laid-Open No. 3-251815). FIG. 34 is adiagram showing the construction of a circuit for stabilizing theoperating point of an optical modulator that implements the currentlyavailable method of compensating the modulator operating point, and FIG.35 is a diagram useful in describing the principle of operating-pointstabilization.

Shown in FIG. 34 are the semiconductor laser diode (DFB-LD) 1, theMZ-type optical modulator (LN optical modulator) 2, the optical fibers 3a, 3 b and a drive circuit 4. An NRZ electric signal (the data signal)is input to the drive circuit 4, which proceeds to generate anelectrical drive signal SD having an amplitude (=Vπ) between theculmination A of light emission and the culmination B of lightextinction in the voltage-optical output characteristic (see FIG. 32B)of the MZ-type optical modulator 2. A low-frequency oscillator 5generates a low-frequency signal SLF having a low frequency f₀ (e.g., 1KHz), a low-frequency superimposing circuit 6 for superimposes alow-frequency signal on the drive signal SD, an optical branching unit 7branches the optical signal from the optical modulator 2, and a lightreceiver (PD) 8 such as a photodiode converts the optical signal outputby the optical modulator 2 to an electrical signal. Numeral 9 denotes anamplifier. A phase comparator 10 detects and outputs a phase differenceθ between the low-frequency signal component of the frequency f₀contained in the optical signal output by the optical modulator 2 andthe low-frequency signal output by the low-frequency oscillator 5. Alow-pass filter (LPF) 11 rectifies the output signal of the phasecomparator 10, and a bias supply circuit 12 controls the bias voltage,which is applied to a signal electrode, in such a manner that the phasedifference θ will become zero.

The low-frequency superimposing circuit 6 subjects the drive signal ofthe MZ-type optical modulator 2 to amplitude modulation by the signalhaving the low frequency of of, the photodiode 8 converts the outputlight of the optical modulator 2 to an electrical signal, the phasecomparator 10 performs a phase comparison between the low-frequencysignal impressed upon the drive signal and the low-frequency signalcomponent contained in the optical signal, and the bias supply circuit12 controls the bias voltage applied to the signal electrode in such amanner that the phase difference θ will become zero.

The optimum operating points of the MZ-type optical modulator are pointsA and B (see FIG. 35) at which the two levels of the waveform of thedrive signal SD give the maximum and minimum output optical powers. Inthe case that there is no fluctuation in the voltage-optical outputcharacteristic of the MZ-type optical modulator 2. Even if the signalSLF having the low frequency f₀ is impressed upon the drive signal SD,upper and lower envelopes ELU, ELL of the output light do not containthe f₀ component and a frequency component that is twice f₀ appears inthe ideal state (curve a).

On the other hand, if the characteristic curve shifts to the left orright from a to b or from a to c (if the operating point shifts to theleft or right) in the manner illustrated, the upper and lower envelopesELU, ELL of the output light both become signals modulated by the samephase. These signals contain the f₀ component. In addition, the phasesof the upper and lower envelopes ELU, ELL of the output light incharacteristic curve b are the opposite of the phases of the upper andlower envelopes ELU, ELL of the output light in characteristic curve c.

By virtue of the foregoing, the direction in which the operating pointdrifts can be detected by comparing the phase of low-frequency signalSLF superimposed on the drive signal and the phase of the low-frequencysignal component contained in the optical signal. The bias voltage canbe controlled in such a manner that this phase difference will becomezero.

Optical duobinary modulation

In a case where an increase in capacity is intended by TDM, a factor isthat chromatic dispersion (GVD) governs transmission distance.Dispersion tolerance is inversely proportional to the square of the datatransmission speed (the bit rate). A dispersion tolerance that is about800 ps/nm in a 10-Gbps system, therefore, deteriorates to about{fraction (1/16)} of this figure, namely to about 50 ps/nm, in a 40-Gbpssystem. One method of reducing waveform degradation due to chromaticdispersion is optical duobinary modulation. (For example, see A. J.Price et al., “Reduced bandwidth optical digital intensity modulationwith improved chromatic dispersion tolerance”, Electron. Lett., vol. 31,No. 1, pp. 58-59, 1995.)

In comparison with the NRZ modulation scheme, optical duobinarymodulation reduces the bandwidth of the optical signal spectrum to abouthalf thereby it reduces the effects of chromatic dispersion. Forexample, whereas the bandwidth of the optical signal spectrum of a10-Gbps NRZ signal is 10 GHz in terms of frequency and 0.2 nm in termsof wavelength, the bandwidth of the optical signal spectrum of a 10-Gbpsduobinary signal is 5 GHz in terms of frequency and 0.1 nm in terms ofwavelength. Because the velocity of light differs depending uponwavelength, the larger the bandwidth of the spectrum of the opticalsignal, the greater the amount of change in the velocity at which lightpropagates and, hence, the greater the distortion of the waveform causedby long-distance transmission. Accordingly, if the bandwidth of thespectrum of the optical signal can be made small by optical duobinarymodulation, the amount of fluctuation in velocity can be reduced and thedispersion tolerance can be increased.

FIG. 36 is a diagram showing the construction of a modulation apparatusthat relies upon optical duobinary modulation, FIGS. 37A, 37B arediagrams useful in describing the principle of optical duobinarymodulation, and FIGS. 39A, 39B are waveform diagrams of the associatedsignals.

Shown in FIG. 36 are the semiconductor laser diode (DFB-LD) 1 and theMZ-type optical modulator 2 having two signal electrodes for applyingphase modulation to the optical signals in the optical waveguides onboth sides, and drive-signal input terminals for inputting complimentarydrive signals to the signal electrodes.

A precoder 21 encodes a 40-Gbps binary NRZ electrical input signal. AD-type flip-flop (D-FF) 22 extracts and stores the output of theprecoder 21 at a 40-GHz clock and outputs a non-inverted signal D and aninverted signal *D. Phase shifters 23 a, 23 b adjust the output phasesof the flip-flop 22 and apply there outputs to amplitude adjusters 24 a,24 b, respectively. The outputs thereof are applied to electricallow-pass filters 25 a, 25 b, respectively, having a bandwidth that isone-fourth the bit rate (=40 Gbps). Bias adjustment circuits (bias tees)are shown at 26 a, 26 b and terminators at 27 a, 27 b. The binary NRZelectrical input signal encoded by the precoder 21 is made 3-valueelectrical signals S1 and S2 having inverted signs by passage throughthe low-pass filters 25 a, 25 b, and these signals are in turn passedthrough the bias tees 26 a, 26 b, thereby generating complimentary3-value electrical drive signals (push-pull signals) S1′, S2′ that areapplied to the respective ones of the two symmetrical signal electrodesof the MZ-type optical modulator 2.

In the MZ-type optical modulator 2, the driving amplitude necessary toturn the CW light on and off generally is Vπ (see FIG. 37B) based uponthe voltage-optical output characteristic. In optical duobinarymodulation, however, each of the two signal electrodes is subjected topush-pull modulation by the amplitude Vπ. (This is modulation in whichvoltages that are always opposite in sign are applied to the twoelectrodes). The voltage applied to the optical modulator 2 is thevoltage difference (=S1′−S2′) between the input signals S1′ and S2′. Inoptical duobinary modulation, in other words, the MZ-type opticalmodulator 2 is modulated by a driving amplitude 2Vπ, namely an amplitudethat is twice Vπ. Further, the bias voltage (the center voltage of theelectrical signal) is set in such a manner that the optical modulator isdriven between two light-emission culminations A, A on thevoltage-optical output characteristic curve.

The details of optical duobinary modulation will now be described.

As shown in FIG. 38, the precoder 21 includes a NOT gate 21 a forinverting an input signal an, a 1-bit (25 ps) delay gate 21 b, and anEX-OR gate 21 c for outputting a signal c_(n) obtained by taking theexclusive-OR between the preceding output c_(n-1) and the presentinverted input b_(n). If reference is had to a truth table of theinverted signal b_(n), the preceding output signal c_(n-1) of the EX-ORgate and the present output signal c_(n) of the EX-OR gate, we have thefollowing:

(1) c_(n)=c_(n-1) (no change in sign) if b_(n)=“0” holds; and

(2) c_(n) =1−c _(n-1) (sign inverted) if b_(n)=“1” holds.

A low-pass filter 25 a has a bandwidth which is only one-fourth of thebit rate, namely 10 GHz. Consider two successive bits of the inputsignal c_(n). If the input data varies at high speed in the manner “0,1” or “1, 0”, the low-pass filter 25 a cannot follow up this change andoutputs 0.5, which is the level intermediate the 0 and 1 levels. If theinput data is two successive “1”s , namely “1, 1”, the low-pass filter25 a outputs the level 1.0; if the input data is two successive “0”s,namely “0, 0”, the low-pass filter 25 a outputs the level 0.0. Morespecifically, the low-pass filter 25 a:

(3) outputs the 0.0 level in a case where the output c_(n) of theprecoder is successive “0”s (“00”: no change in sign);

(4) outputs the 1.0 level in a case where the output c_(n) of theprecoder is successive “1”s (“11”: no change in sign); and

(5) outputs the 0.5 level in a case where the sign of the output c_(n)of the precoder reverses (“01” or “10”).

From (1) to (5) above, the output of the low-pass filter 25 a changes ifthe sign of the precoder output changes. That is, the low-pass filter 25a outputs the 0.0 or +1.0 level as the output d_(n) if the input data anis “1”, and outputs the +0.5 level as the output d_(n) if the input dataa_(n) is “0”. Similarly, the low-pass filter 25 b outputs the 0.0 or−1.0 level as the output *d_(n) if the input data a_(n) is “1”, andoutputs the 0.5 level as the output *d_(n) if the input data a_(n) is“0”. Accordingly, if the level ±1.0 is ±Vπ and the level ±0.5 is ±Vπ/2,then 2Vπ or 0 is input across the signal electrodes of the MZ-typeoptical modulator 2 when the input data an is “1” and Vπ is input acrossthe signal electrodes of the MZ-type optical modulator 2 when the inputdata an is “0”. As a result, with reference to FIG. 37B,

(1) “1” is output (light is emitted) if the input data a_(n) is “1”, atwhich value 2Vπ or 0 is input across the signal electrodes of theMZ-type optical modulator 2; and

(2) “0” is output (light is extinguished) if the input data a_(n) is“0”, at which value Vπ is input across the signal electrodes of theMZ-type optical modulator 2.

Thus, the waveforms of the output signals S1, S2 from the low-passfilters 25 a, 25 b are as shown in FIG. 39A, and the optical signaloutput S3 from the MZ-type optical modulator 2 becomes as shown in FIG.39B.

The characterizing feature of the optical duobinary modulation method isthat the bandwidth of the optical signal spectrum is approximately halfthat obtained with the conventional NRZ modulation method describedabove. This makes it possible to reduce the effects of chromaticdispersion.

Further, in accordance with optical duobinary modulation, channels canbe disposed at higher density in the WDM scheme. In a case where theintent is to enlarge capacity by the WDM technique, bandwidth ofwavelength at which a optical amplifier can amplify are limitingfactors. However, if optical duobinary modulation is used, the fact thatthis method provides a narrow bandwidth for the optical signal spectrumcan be utilized and channels can be disposed at a higher density withinthe amplification bandwidth of the light amplifier.

Further, in optical duobinary modulation, chirping can be reducedbecause of push-pull drive. Chirping occurs and the direction thereofreverses when the applied voltage of an optical modulator increases anddecreases. With optical duobinary modulation, however, the electrodesare driven by mutually complimentary electrical signals. Consequently,when the applied voltage increases at one electrode, it decreases at theother, and when the applied voltage decreases at one electrode, itincreases at the other. Since the optical phase of the output opticalsignal is the sum of the optical phases produced at the two electrodes,chirping is reduced by cancellation.

An advantage of the MZ-type optical modulator is the fact thattransmitted light experiences little chirping, as mentioned above.However, a change in the temperature of the LiNbO₃ constituting thesubstrate and the aging thereof are accompanied by temporal drift of theoperating point of the voltage-optical output characteristic.

For this reason, it is necessary to control the bias voltage independence upon drift of the operating point, just is in the NRZmodulation scheme, in optical duobinary modulation as well. However, theproblems set forth below arise when the operating-point compensationtechnique of NRZ modulation is applied directly to optical duobinarymodulation. FIG. 40 is a diagram useful in describing a case where theoperating-point compensation technique of NRZ modulation is applieddirectly to optical duobinary modulation.

With optical duobinary modulation, the driving voltage is made twicethat used in NRZ modulation. Consequently, if the voltage-optical outputcharacteristic shifts to the left or right from the ideal characteristica to b or c, the envelopes ELU, ELL of the optical signal correspondingto the ON-side and OFF-side portions EU and EL of the electrical drivingsignal of the modulator subjected to low-frequency modulation take onmutually opposite phases and cancel each other out, making it impossibleto detect the signal component of the low frequency f₀. The problem thatarises, therefore, is that the ABC control method employed in theconventional NRZ modulation method cannot be applied to a modulationscheme, which includes optical duobinary modulation, wherein an opticalmodulator is driven between two light-emission culminations or betweentwo light-extinction culminations of the voltage-optical outputcharacteristic.

Another problem is that the conventional ABC control method only assumesuse of an MZ-type optical modulator configured for electrode drive onone side. This means that it is necessary to also consider setting of anoperating point in a case where an optical modulator configured fordriving electrodes on both sides is used in optical duobinarymodulation, NRZ modulation and RZ modulation.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to make it possibleto compensate for drift of the operating point that accompanies avariation in the voltage-optical output characteristic of an opticalmodulation apparatus in which an optical modulator is driven by theamplitude between two light-emission culminations or twolight-extinction culminations of the voltage-optical outputcharacteristic.

Another object of the present invention is to so arrange it that theoperating point can be controlled to assume the proper position even ifthe voltage-optical output characteristic of the optical modulatorvaries in a case where the optical modulator, which is configured fordriving electrodes on both sides, is used in optical duobinarymodulation, NRZ modulation and RZ modulation.

According to a first aspect of the present invention, when an opticalmodulator having a voltage-optical output characteristic in whichoptical output varies periodically with respect to a voltage value of anelectrical drive signal is driven by the electrical drive signal, whichhas an amplitude (=2Vπ) between two light-emission culminations or twolight extinction culminations of the voltage-optical outputcharacteristic, (1) a prescribed low-frequency signal is superimposed onthe drive signal, (2) operating-point drift of the optical modulator isdetected based upon the low-frequency signal component contained in anoptical signal output by the optical modulator, and (3) the operatingpoint of the optical modulator is controlled in dependence upon theoperating-point drift (NRZ modulation, RZ modulation).

According to a second aspect of the present invention, two mutuallycomplimentary drive signals having an amplitude between a light-emissionculmination and a neighboring light-extinction culmination of avoltage-optical output characteristic of an optical modulator aregenerated, a low-frequency signal is superimposed on at least one ofthese complimentary drive signals, and the drive signals are input tothe signal electrodes to drive electrodes on both sides of the opticalmodulator (optical duobinary modulation).

In the first and second aspects of the present invention, the opticalmodulator is an optical modulator, e.g., an MZ-type optical modulator,having optical waveguides that branch on a light input side and merge ona light output side, two signal electrodes for applying phase modulationto optical signals in the branched optical waveguides on both sides, andtwo drive-signal input terminals for inputting complimentary drivesignals to respective ones of the signal electrodes.

Further, in the first and second aspects of the present invention,examples of methods of superimposing a low-frequency signal on a drivesignal are:

(1) superimposing the low-frequency signal on the drive signal in such amanner that phases of upper and lower envelopes of the drive signalcoincide;

(2) superimposing the low-frequency signal on the drive signal in such amanner that only an upper or a lower envelope of the drive signalvaries;

(3) superimposing the low-frequency signal on the drive signal in such amanner that amplitudes of upper and lower envelopes of the drive signaldiffer;

(4) superimposing the low-frequency signal on the drive signal in such amanner that frequencies of upper and lower envelopes of the drive signaldiffer; and

(5) superimposing the low-frequency signal on the drive signal in such amanner that phases of upper and lower envelopes of the drive signaldiffer.

In accordance with the first and second aspects of the present inventionas described above, a low-frequency signal component can be detectedfrom an optical signal output by an optical modulator, andoperating-point drift that accompanies fluctuation of thevoltage-optical output characteristic of the optical modulator can becompensated for by a simple arrangement. Further, in accordance withoptical duobinary modulation of the second aspect of the presentinvention, the influence of chromatic dispersion can be reduced andchirping can be diminished by push-pull drive.

According to a third aspect of the present invention, an opticalmodulator having optical waveguides that branch on a light input sideand merge on a light output side, two signal electrodes for applyingphase modulation to optical signals in the optical waveguides on bothsides, two drive-signal input terminals for inputting complimentarydrive signals to respective ones of the signal electrodes, and avoltage-optical output characteristic that varies periodically, isdriven by a drive signal that has an amplitude (=Vπ) between alight-emission culmination and a neighboring light extinctionculmination of the voltage-optical output characteristic. At this time,(1) complimentary drive signals whose amplitude is one-half of theamplitude (=Vπ) are generated, (2) a prescribed low-frequency signal issuperimposed on one of the complimentary drive signals, and (3)operating-point drift of the optical modulator is detected based uponthe low-frequency signal component contained in an optical signal outputby the optical modulator, and the operating point of the opticalmodulator is controlled in dependence upon the operating-point drift.

The third aspect of the present invention is such that when the opticalmodulator is driven by the drive signal that has an amplitude Vπ betweenthe light-emission culmination and the neighboring light extinctionculmination of the voltage-optical output characteristic, twocomplimentary drive signals of amplitude Vπ/2 are generated and theoptical modulator is subjected to push-pull drive by these complimentarydrive signals. As a result, chirping can be reduced. Moreover, thelow-frequency signal component can be detected reliably from the opticalsignal output by the optical modulator, thereby making it possible tocompensate for drift of the operating point.

In accordance with the first through third inventions, as describedabove, the low-frequency signal component can be detected reliably fromthe optical signal output of the optical modulator by way of a simplearrangement, thereby making it possible to compensate foroperating-point drift that accompanies variation of the voltage-opticaloutput characteristic of the optical modulator, even in a case where anoptical modulator configured for drive on both sides is used in opticalduobinary modulation, NRZ modulation and RZ modulation.

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the principle underlying an opticalmodulation apparatus according to the present invention;

FIG. 2 is a diagram useful in describing the principle underlying afirst method according to the present invention (same-phaselow-frequency modulation applied to ON and OFF sides of an electricaldrive signal);

FIG. 3 is a diagram useful in describing the principle underlying asecond method according to the present invention (low-frequencymodulation applied only to the ON side of an electrical drive signal);

FIG. 4 is a diagram useful in describing the principle underlying athird method according to the present invention (low-frequencymodulation of different amplitudes applied to ON and OFF sides of anelectrical drive signal);

FIG. 5 is a diagram useful in describing the principle underlying afourth method according to the present invention (low-frequencymodulation of different frequencies applied to ON and OFF sides of anelectrical drive signal);

FIG. 6 is a diagram useful in describing the principle underlying afifth method according to the present invention (low-frequencymodulation of different phases applied to ON and OFF sides of anelectrical drive signal);

FIG. 7 is a diagram showing the construction of an optical modulationapparatus according to a first embodiment;

FIG. 8 is a waveform diagram of signals associated with FIG. 7;

FIG. 9 is a first modification of the optical modulation apparatusaccording to the first embodiment;

FIG. 10 is a waveform diagram of signals associated with FIG. 9;

FIG. 11 is a second modification of the optical modulation apparatusaccording to the first embodiment;

FIG. 12 is a waveform diagram of signals associated with FIG. 11;

FIG. 13 is a diagram showing the construction of an optical modulationapparatus according to a second embodiment;

FIG. 14 is a waveform diagram of signals associated with FIG. 13;

FIG. 15 is a modification of the optical modulation apparatus accordingto the second embodiment;

FIG. 16 is a waveform diagram of signals associated with FIG. 15;

FIG. 17 is a diagram showing the construction of an optical modulationapparatus according to a third embodiment;

FIG. 18 is a waveform diagram of signals associated with FIG. 17;

FIG. 19 is a diagram showing the construction of an optical modulationapparatus according to a fourth embodiment;

FIG. 20 is a waveform diagram of signals associated with FIG. 19;

FIG. 21 is a diagram showing the construction of an optical modulationapparatus according to a fifth embodiment;

FIG. 22 is a waveform diagram of signals associated with FIG. 21;

FIG. 23 is a diagram showing the construction of an optical modulationapparatus according to a sixth embodiment;

FIG. 24 is a waveform diagram of signals associated with FIG. 23;

FIG. 25 is a diagram showing the construction of an optical modulationapparatus according to a seventh embodiment;

FIG. 26 is a diagram showing the construction of an optical modulationapparatus according to an eighth embodiment;

FIG. 27 is a waveform diagram of signals associated with FIG. 26;

FIGS. 28A and 28B are diagrams useful in describing switching of a biaspoint of a modulator;

FIG. 29 is a diagram showing the construction of an optical modulationapparatus according to a ninth embodiment;

FIGS. 30A and 30B are diagrams useful in describing a case where a lightreceiver is incorporated in a substrate;

FIG. 31 is a diagram showing the construction of an optical modulatorcapable of modulating any polarized light wave;

FIGS. 32A and 32B are diagrams for describing a Mach-Zehnder opticalmodulator;

FIG. 33 is a diagram useful in describing the problems caused by driftof the operating point of an optical modulator;

FIG. 34 is a diagram showing the construction of a circuit forstabilizing the operating point of an optical modulator in NRZmodulation;

FIG. 35 is a diagram showing the principle underlying the circuit forstabilizing the operating point of an optical modulator in NRZmodulation;

FIG. 36 is a diagram showing an example of the construction of amodulator using optical duobinary modulation;

FIGS. 37A and 37B are diagrams for describing the principle of opticalduobinary modulation;

FIG. 38 is another diagram for describing the principle of opticalduobinary modulation;

FIGS. 39A and 39B are waveform diagrams showing signals associated withthe optical duobinary modulator; and

FIG. 40 is a diagram useful in describing a case where a techniquesimilar to that of NRZ modulation is applied to optical duobinarymodulation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(A) Overview of the invention

(a) Basic construction

FIG. 1 is a diagram showing the basic construction of a first opticalmodulation apparatus according to the present invention. Shown in FIG. 1are a semiconductor laser (DFB-LD) 51, an optical modulator (e.g., anMZ-type optical modulator) 52 the voltage-optical output characteristicwhereof varies periodically, a drive signal generator 53 for generatingelectrical drive signals SD, SD′ that drive the optical modulator by anamplitude 2·Vπ between two light-emission culminations A, A or two lightextinction culminations B, B of the voltage-optical outputcharacteristic, a low-frequency oscillator 54 for generating aprescribed low-frequency signal, a low-frequency superimposing unit 55for superimposing the low-frequency signal on the drive signal SD, anoptical branching unit 56 for branching an optical signal output by theoptical modulator 52, a low-frequency signal detector 57 for detectingthe low-frequency signal component contained in an optical signal outputby the optical modulator and detecting operating-point drift of theoptical modulator based upon the low-frequency signal component, and anoperating-point control unit 58 for controlling the position of theoperating point by controlling the bias voltage of the optical modulatorin dependence upon the direction of drift of the operating point of theoptical modulator.

When the optical modulator 52 is driven by the electrical signal havingthe amplitude 2·Vπ, the low-frequency superimposing unit 55 superimposesa low-frequency signal SLF on the electrical drive signal SD output bythe drive signal generator 53. The low-frequency signal detector 57detects the low-frequency signal component contained in the opticalsignal output by the optical modulator 52, and the operating-pointcontrol unit 58 discriminates the direction of operating-point driftbased upon this detected low-frequency signal component and controls thebias voltage of the optical modulator 52. More specifically, theoperating-point control unit 58 controls the operating point in such amanner that the center level of the electrical drive signal (themodulator driving voltage signal) applied to the modulator will coincidewith the level of the extinction culmination B of the characteristiccurve and the levels on both sides of the electrical drive signal willcoincide with the light-emission culminations A, A of the characteristiccurve.

(b) Method of superimposing low-frequency signal

Methods of superimposing the low-frequency signal on the drive signalare as follows:

(1) a first method (FIG. 2) of superimposing the low-frequency signalSLF on the drive signal SD in such a manner that the phases of upper andlower envelopes EU, EL of the modulator driving voltage signal (theinput electrical drive signal of the modulator) will coincide;

(2) a second method (FIG. 3) of superimposing the low-frequency signalSLF on the drive signal SD in such a manner that only the upper envelopeEU or lower envelope EL of the modulator driving voltage signal willvary;

(3) a third method (FIG. 4) of superimposing the low-frequency signalSLF on the drive signal SD in such a manner that the amplitudes of theupper and lower envelopes EU and EL of the modulator driving voltagesignal will differ; and

(4) a fourth method (FIG. 5) of superimposing the low-frequency signalSLF on the drive signal SD in such a manner that the frequencies of theupper and lower envelopes EU and EL of the modulator driving voltagesignal will differ; and

(5) a fifth method (FIG. 6) superimposing the low-frequency signal SLFon the drive signal SD in such a manner that the phases of the upper andlower envelopes EU and EL of the modulator driving voltage signal willdiffer.

As shown in FIG. 2, the first method is a method of performinglow-frequency modulation in such a manner that the envelopes EU and ELcorresponding to the ON and OFF sides, respectively, of the modulatordriving voltage signal take on the same phase. The optimum operatingpoints of the MZ-type optical modulator 52 are the points A, A at whichthe two levels of the waveform of the modulator driving voltage signalgive the maximum output optical power and the point B at which theintermediate level gives the minimum output optical power. In the casethat there is no fluctuation in the voltage-optical outputcharacteristic of the MZ-type optical modulator 52. Even if the signalSLF having the low frequency f₀ is impressed upon the modulator drivingvoltage signal, the upper and lower envelopes ELU, ELL of the outputlight do not contain the f₀ component and a frequency component that istwice f₀ appears in the ideal state (curve a).

On the other hand, if the characteristic curve shifts from a to the leftor right, as indicated by b, c (if the operating point shifts to theleft or right), the upper and lower envelopes ELU, ELL of the outputlight will contain the f₀ component. In this case, the envelopes EU andEL on the ON and OFF sides, respectively, of the modulator drivingvoltage signal are made identical in phase, whereby the envelopes ELU,ELL of the optical signal are made identical in phase, unlike thesituation illustrated in FIG. 40. As a consequence, the f₀ component isnot canceled out and can be detected reliably. Moreover, the phase ofthe envelopes ELU, ELL of the output light is inverted depending uponthe direction in which the characteristic curve shifts. This means thatsignal component of the superimposed low frequency f₀ can be detectedeven if the voltage-optical output characteristic of the modulatorshifts to the left or right from the ideal curve a to the curve b or c,i.e., even if the operating point varies from the optimum point.Further, since the phase of the signal of the f₀ component differs by180° depending upon the direction in which the operating point drifts,the direction in which the operating point drifts can be detected bycomparing this phase with the phase of the low-frequency signal SLFsuperimposed upon the electrical drive signal SD.

As shown in FIG. 3, the second method is a method of performinglow-frequency modulation only on the ON side (or on the OFF side). Thesecond method is such that if the characteristic curve shifts from a tothe left or right, as indicated by b, c, only the envelope ELU on theupper side of the output light will contain the f₀ component. As aresult, the low-frequency signal component can be detected reliably.Moreover, the phase of the envelope ELU of the output light is inverteddepending upon the direction of shift. This means that signal componentof the superimposed low frequency f₀ can be detected even if thevoltage-optical output characteristic of the modulator shifts to theleft or right from the ideal curve a to the curve b or c, i.e., even ifthe operating point varies from the optimum point. Further, since thephase of the signal of the f₀ component differs by 180° depending uponthe direction in which the operating point drifts, the direction inwhich the operating point drifts can be detected by comparing this phasewith the phase of the low-frequency signal SLF superimposed upon theelectrical drive signal SD.

As shown in FIG. 4, the third method is a method of performing amplitudemodulation in such a manner that the envelopes EU and EL correspondingto the ON and OFF sides of the modulator driving voltage signal take ondifferent amplitudes. If the voltage-optical output characteristic curveshifts from a to the left or right, as indicated by b, c, the upper andlower envelopes ELU, ELL of the output light will contain the f₀component. In this case, the envelopes EU and EL on the ON and OFFsides, respectively, of the modulator driving voltage signal areopposite in phase, and therefore the phases of the envelopes ELU, ELL ofthe optical signal also are opposite in phase. However, since theamplitudes of the envelopes EU, EL are different, the signal obtained bycombining the envelopes ELU, ELL of the optical signal does not becomezero and the f₀ component can be detected reliably. Moreover, the phaseof the signal obtained by combining the envelopes ELU, ELL of the outputlight is inverted depending upon the direction of shift. This means thatthe signal component of the superimposed low frequency f₀ can bedetected even if the voltage-optical output characteristic of themodulator shifts to the left or right from the ideal curve a to thecurve b or c, i.e., even if the operating point varies from the optimumpoint. Further, since the phase of the signal of the f₀ componentdiffers by 180° depending upon the direction in which the operatingpoint drifts, the direction in which the operating point drifts can bedetected by comparing this phase with the phase of the low-frequencysignal SLF superimposed upon the electrical drive signal SD.

As shown in FIG. 5, the fourth method is a method of obtaining differentfrequencies f₁, f₂ of low-frequency modulation of the envelopes EU andEL corresponding to the ON and OFF sides of the modulator drivingvoltage signal. If the voltage-optical output characteristic curveshifts from a to the left or right, as indicated by b, c, the upper andlower envelopes ELU, ELL of the output light will contain the f₁, f₂components, respectively. As a consequence, these signal components canbe detected reliably. Moreover, the phases of the envelopes ELU, ELL ofthe output light are inverted depending upon the direction in which theoperating point shifts. This means that signal components of thesuperimposed frequencies f₁, f₂ can be detected from the output light ofthe modulator even if the voltage-optical output characteristic of themodulator shifts to the left or right from the ideal curve a to thecurve b or c. Further, since the phases of the signal components of thefrequencies f₁, f₂ differ by 180° depending upon the direction in whichthe operating point drifts, this direction can be detected.

As shown in FIG. 6, the fifth method is a method of obtaining differentphases of low-frequency modulation of the envelopes EU and ELcorresponding to the ON and OFF sides of the modulator driving voltagesignal. If the voltage-optical output characteristic curve shifts from ato the left or right, as indicated by b, c, the upper and lowerenvelopes ELU, ELL of the output light will contain the f₀ component. Inthis case, the phases of the envelopes EU, EL on the ON side and OFFside of the modulator driving voltage signal are offset by θ, andtherefore the signal obtained by combining the envelopes ELU, ELL of theoptical signal does not become zero and the f₀ component can be detectedreliably. Moreover, the phase of the signal obtained by combining theenvelopes ELU, ELL of the output light is inverted depending upon thedirection of shift. This means that the signal component of thesuperimposed low frequency f₀ can be detected even if thevoltage-optical output characteristic of the modulator shifts to theleft or right from the ideal curve a to the curve b or c. Further, sincethe phase of the signal of the f₀ component is inverted depending uponthe direction in which the operating point drifts, this direction can bedetected.

(c) Optical modulator configured for electrode drive on both sides

The optical modulator is not defined above. Used as the opticalmodulator 52 is an LN optical modulator (MZ-type optical modulator)configured for electrode drive on both sides, having (1) opticalwaveguides 52 a, 52 b that are formed on the LiNbO₃ substrate and branchon a light input side and merge on a light output side; (2) two signalelectrodes 52 c, 52 d for applying phase modulation to optical signalsin the branched optical waveguides on both sides; (3) two drive-signalinput terminals 52 e, 52 f for inputting complimentary drive signals torespective ones of the signal electrodes; and (4) a bias-voltage inputterminal 52 g.

In a case where use is made of such an optical modulator having drivenelectrodes on both sides thereof, the drive signal generator 53generates two mutually complimentary drive signals (push-pull drivesignals) SD, SD′ having an amplitude Vπ between a light-emissionculmination A and a neighboring light-extinction culmination B of thevoltage-optical output characteristic of the optical modulator aregenerated, and the low-frequency superimposing unit 55 superimposes thelow-frequency signal SLF on at least one of the electrical drivesignals, i.e., the drive signal SD, and inputs the resulting signal tothe signal electrode 52 c. The other drive signal SD′ is input to thesignal electrode 52 d. Thus, both sides of the modulator are driven. Itshould be noted that the above-described low-frequency signalsuperimposing method can be applied also in a case where the opticalmodulator driven on both its sides is used in NRZ modulation and RZmodulation, etc.

(B) Embodiment

(a) First embodiment

FIG. 7 is a diagram showing the construction of an optical modulationapparatus according to a first embodiment. This is an example in whichan LN optical modulator (MZ-type optical modulator) configured for driveon both sides (i.e., the modulator has driven electrodes on both sides)is used as the optical modulator and low-frequency modulation is carriedout in such a manner that envelopes on the ON and OFF sides,respectively, of the modulator driving voltage signal applied to theoptical modulator will take on the same phase (see FIG. 2 showing theprinciple underlying the first method of the present invention). FIG. 8is a waveform diagram of signals associated with FIG. 7.

Shown in FIG. 7 are the semiconductor laser (DFB-LD) 51, and the MZ-typeoptical modulator 52 the voltage-optical output characteristic whereofvaries periodically. The optical modulator 52 includes the opticalwaveguides 52 a, 52 b that are formed on the LiNbO₃ substrate and branchon the light input side and merge on the light output side, the twosignal electrodes 52 c, 52 d for applying phase modulation to opticalsignals in the branched optical waveguides on both sides, and twodrive-signal input terminals 52 e, 52 f for inputting complimentarydrive signals to respective ones of the signal electrodes, andbias-voltage input terminals 52 g, 52 h for inputting bias voltages tothe signal electrodes.

The arrangement further includes the drive signal generator 53 whichgenerates the two mutually complimentary drive signals (push-pull drivesignals) SD, SD′ having the amplitude Vπ between the light-emissionculmination A and the neighboring light-extinction culmination B of thevoltage-optical output characteristic (see FIG. 1) of the opticalmodulator 52. The drive signal generator 53, which corresponds to thecircuitry from the precoder 21 to the low-pass filters 25 a, 25 b ofFIG. 36, converts the binary input data to the 3-value push-pull drivesignals SD, SD′ and outputs these signals. The drive signal SD is a3-value signal, namely a signal having a Vπ or 0 level if the input datais “1” and a Vπ/2 level if the input data is “0”. The drive signal SD′is a 3-value signal having a −Vπ or 0 level if the input data is “1” anda −Vπ/2 level if the input data is “0”.

The arrangement further includes the low-frequency oscillator 54 forgenerating the prescribed low-frequency signal SLF, e.g., a frequency f₀of 1 KHz, and the low-frequency superimposing unit 55, which isconstituted by a coil L for passing a low-frequency signal and acapacitor C1 for cutting direct current, for superimposing thelow-frequency signal on one drive signal, namely the drive signal SD.The low-frequency superimposing unit 55 uses a bias tee to vary thecenter voltage of the drive signal SD by the low-frequency signal offrequency f₀ at the input side of the optical modulator 52. The opticalbranching unit 56 branches the optical signal output by the MZ-typeoptical modulator 52. A light receiver 57 a such as a photodiodeconverts the branched light to an electrical signal, an amplifyingcircuit 57 b amplifies the output of the photodiode 57 a, a phasecomparator 57 c to which the low-frequency signal SLF output by thelow-frequency oscillator 54 and an electrical signal conforming to thephotodiode output are input detects the low-frequency signal componentcontained in the photodiode output by a phase comparison and outputs thedetected low-frequency signal component as a signal indicative of thedrift of the operating point of the modulator, and a low-pass filter 57d smoothens the output of the phase comparator. The photodiode 57 a,amplifying circuit 57 b, phase comparator 57 c and low-pass filter 57 dconstruct the low-frequency signal detector 57 of FIG. 1 that detectsdrift of the operating point of the optical modulator. In order to raisethe precision of the phase comparator, a bandpass filter for thefrequency f₀ can be inserted on the output side of the amplifyingcircuit 57 b.

The bias supply circuit (operating-point control unit) 58, which isconstituted by a bias tee 58 a and a 50-Ω terminator 58 b, controls theposition of the operating point by controlling the bias voltage Vb1applied to the signal electrode 52 a in dependence upon the direction ofdrift of the operating point of the optical modulator. The bias tee 58 ahas a coil L, which is for supplying the signal electrode 52 a of theoptical modulator with the bias voltage Vb1, and a capacitor C forinputting a high-frequency signal from the modulator to the terminator58 b. A bias tee 59, which consists of a coil L and capacitor C,supplies the other signal electrode 52 b of the modulator with a biasvoltage Vb2. A terminator 60 is connected to the bias tee 59. Drivecircuits 61, 62 input the drive signals SD, SD′, which are output by thedrive signal generator 53, to the respective signal electrodes of theoptical modulator 52, thereby driving the modulator.

In the first embodiment, the amplitude of the modulator driving voltagesignal applied to the optical modulator 52 is 2·Vπ (the voltage betweenthe two light-emission culminations A, A of the voltage-optical outputcharacteristic). As a result, the modulator performs push-pullmodulation, in which drive signals [see (a) and (d) in FIG. 8] ofmutually inverted amplitude Vπ output by the drive circuits 61, 62 areinput to the optical modulator 52. Chirping of the optical modulatedsignal is made zero by this push-pull modulation and degradation of thetransmitted waveform can be reduced.

The voltage plotted along the horizontal axis of the voltage-opticaloutput characteristic (FIG. 1) of the optical modulator 52 is not theabsolute value of the potentials of both electrodes, but is thepotential difference between these electrodes. The bias voltage Vb2corresponding to the drive circuit 62 therefore is fixed at zero (or atanother constant voltage) using the bias tee 59. Only the bias voltageVb1 corresponding to the drive circuit 61 is controlled based uponoperating-point drift. Further, low-frequency amplitude modulation isperformed using the low-frequency signal SLF, which is output by thelow-frequency oscillator 54, superimposed solely on the drive signalfrom the drive circuit 61 by the low-frequency superimposing unit 55.

Capacitances C1, C2 and C3 interrupt the bias voltages applied to thesignal electrodes of the optical modulator at the indicated positions.It is required that the capacitance C3 be made a sufficiently largevalue so as to be capable of passing the low-frequency signal SLF.

The centers of the output signals from the low-frequency superimposingunit 55 and drive circuit 62 agree with the bias voltages Vb1, Vb2 (=0V) of the optical modulators 52 a, 52 b, and therefore the signalwaveforms become as shown in (c) and (e) of FIG. 8. As a result, themodulator driving voltage signal applied to the optical modulator 52 hasthe amplitude 2·Vπ [indicated by (c)−(e) in FIG. 8], which correspondsto the potential difference across both electrodes, as well as theenvelopes EU, EL, on the ON and OFF sides, modulated by the lowfrequency f₀ and at the same phase.

If the operating point of the optical modulator 52 drifts from theoptimum value, a low-frequency signal component having a phaseconforming to the direction of drift is produced in the optical signaloutput by the optical modulator 52. From this point onward, therefore,the bias voltage Vb1 of the optical modulator is controlled in adirection that will cancel out this low-frequency component. Morespecifically, the optical branching unit 56 branches part of the opticalsignal output from the optical modulator 52, the photodiode 57 aconverts this optical signal to an electrical signal, and the amplifyingcircuit 57 b inputs the electrical signal to the phase comparator 57 cupon amplifying the signal to the required amplitude. The phasecomparator 57 c, to which the low-frequency signal SLF output by thelow-frequency oscillator 54 and the electrical signal conforming to thephotodiode output are input, extracts the low-frequency signal from thephotodiode output by a phase comparison and inputs the extractedlow-frequency signal to the bias supply circuit 58. The latter controlsthe bias voltage Vb1 in such a direction that the low-frequency signalcomponent in the photodiode output will be come zero.

FIG. 7 illustrates a method in which low frequency is superimposedsolely upon the drive circuit 61. However, it is also possible tosubject the drive signals from both drive circuits 61, 62 to similarlow-frequency amplitude modulation simultaneously at phases that are theopposite of each other. In such case the low-frequency modulatedamplitude of the modulator driving voltage signal indicated at (c)-(e)in FIG. 8 would double.

FIG. 9 illustrates a first modification of the optical modulationapparatus according to the first embodiment. Components identical withthose shown in FIG. 7 are designated by like reference characters. Inthe first embodiment, the center voltage of the drive signal SD isvaried by the low-frequency signal of frequency f₀ at the input side ofthe optical modulator 52. In this modification, however, the biasvoltage input to the signal electrode 52 c can be varied by thelow-frequency signal of frequency f₀. The modification of FIG. 9 differsfrom the first embodiment in the following respects:

(1) The output terminal of the low-frequency oscillator 54 and theoutput terminal of the bias supply circuit 58 are connected to vary thebias voltage Vb1 by the low-frequency signal of frequency f₀.

(2) The bias voltage Vb1, the amplitude of which is varied by thelow-frequency, is input to the signal electrode 52 c of the opticalmodulator 52 via the low-frequency superimposing unit 55. Thecapacitances C1, C2 and C3 interrupt the bias voltages, which areapplied to the signal electrodes of the optical modulator, at theindicated positions, thereby preventing input of these voltages to thedrive circuits and low-frequency oscillator.

The signal waveforms associated with the arrangement of FIG. 9 areidentical with those of the first embodiment, as illustrated in FIG. 10.That is, the modulator driving voltage signal [see (c)-(e) in FIG. 10]applied to the optical modulator 52 has the amplitude 2·Vπ as well asthe envelopes EU, EL, on the ON and OFF sides, both modulated by the lowfrequency f₀ and at the same phase. Subsequent control of the operatingpoint is the same as in the first embodiment.

FIG. 11 illustrates a second modification of the optical modulationapparatus according to the first embodiment. Components identical withthose shown in FIG. 7 are designated by like reference characters. Inboth the first embodiment and first modification, the signal electrodesof the optical modulator 52 that apply the modulating signals are commonwith the bias electrodes that apply the center voltage. However, theseelectrodes can be provided as separate electrodes for drive signals andfor the bias voltages. Providing these electrodes as separate electrodesmakes it possible to eliminate the capacitances C1, C2 for interruptingthe bias voltages.

The modification of FIG. 11 differs from the first embodiment in thefollowing respects:

(1) The output terminal of the low-frequency oscillator 54 and theoutput terminal of the bias supply circuit 58 are connected to vary thebias voltage Vb1 by the low-frequency signal of frequency f₀.

(2) The electrode 52 c is separated into electrodes 52c₁, 52c₂ for thedrive signal and bias voltage, respectively, and the electrode 52 d isseparated into electrodes 52d₁, 52d₂ for the drive signal and biasvoltage, respectively.

(3) The drive signals output by the drive circuits 61, 62 are input tothe signal electrodes 52c₁, 52d₁, respectively.

(4) The bias voltage Vb1 whose amplitude is varied by the low-frequencysignal is input to the bias voltage electrode 52c₂ of the opticalmodulator 52 via the low-frequency superimposing unit 55, and the biasvoltage Vb2 (=0) is input to the bias voltage electrode 52d₂.

(5) The capacitances C1 and C2 are deleted.

The bias voltage output by the low-frequency superimposing unit 55 has awaveform upon which the low-frequency signal is superimposed, as shownat (c) in FIG. 8. As a result of separately providing the electrodes forthe drive signals and the electrodes for the bias voltages, a modulatordriving voltage signal indicated at (a)+(c)−(d) in FIG. 12 enters theoptical modulator 52. The modulator driving voltage signal possesses awaveform having the amplitude 2·Vπ as well as the envelopes EU, EL, onthe ON and OFF sides, modulated by the low frequency f₀ and at the samephase.

In FIG. 11, the electrodes in the arrangement of the first modification(FIG. 9) are separated into electrodes for drive signals and electrodesfor bias voltages. However, the electrodes in the arrangement of thefirst embodiment shown in FIG. 7 can also be separated into electrodesfor drive signals and electrodes for bias voltages.

(b) Second embodiment

FIG. 13 is a diagram showing the construction of an optical modulationapparatus according to a second embodiment, and FIG. 14 shows theassociated signal waveforms. The second embodiment differs from thefirst embodiment in the method of superimposing the low-frequencysignal. Components identical with those shown in FIG. 7 are designatedby like reference characters.

According to the first embodiment, the arrangement is such that theinput side of the optical modulator 52 is provided with thelow-frequency superimposing unit 55 to vary the center voltage of thedrive signal SD by the low-frequency signal. According to the secondembodiment, however, the gains of the drive circuits 61, 62 are variedby a low-frequency signal, thereby amplitude modulating the drivesignals by the low-frequency signal.

Shown in FIG. 13 are the semiconductor laser (DFB-LD) 51, and theMZ-type optical modulator 52, the drive signal generator 53 whichgenerates the two mutually complimentary drive signals (push-pull drivesignals) SD, SD′ [(a), (e) in FIG. 14] having the amplitude Vπ, thelow-frequency oscillator 54 for generating the low-frequency signal SLFof frequency f₀, and an amplitude modulating signal generator 55′, towhich the low-frequency SLF is input, for generating two amplitudemodulating signals SAM₁, SAM₂ [(c), (f) in FIG. 14] whose phases aredisplaced from each other by 180°. The amplitude modulating signalgenerator 55′ functions as low-frequency superimposing means forsuperimposing a low-frequency signal upon the drive signals SD, SD′.

Also shown in FIG. 13 are the optical branching unit 56 for branchingthe optical signal output by the MZ-type optical modulator 52, thephotodiode 57 a, the amplifying circuit 57 b for amplifying the outputof the photodiode 57 a, the phase comparator 57 c for detecting thelow-frequency signal component contained in the photodiode output andoutputting the low-frequency signal component as a signal indicative ofthe drift of the operating point of the modulator, and the low-passfilter 57 d for smoothing the output of the phase comparator. Also shownare the bias supply circuit (operating-point controller) 58 forcontrolling the position of the operating point by controlling the biasvoltage Vb1, which is applied to the signal electrode 52 c, based uponthe low-frequency signal component contained in the photodiode output,namely the drift of the operating point of the optical modulator 52. Thedrive circuits 61, 62 input the drive signals SD, SD′, which are outputby the drive signal generator 53, to the respective signal electrodes 52c, 52 d of the optical modulator 52, thereby driving the modulator. Thedrive circuits 61, 62 have gain control terminals to which the amplitudemodulating signals SAM₁, SAM₂, respectively, from the amplitudemodulating signal generator 55′ are applied. The capacitances C1, C2interrupt the bias voltages, which are applied to the respective signalelectrodes of the modulator, at the positions they occupy.

By mutually inverting the amplitude modulating signals SAM₁, SAM₂, whichare applied to the drive circuits 61, 62, respectively, in the mannershown at (c) and (f) in FIG. 14, the drive circuits 61 and 62 are madeto output drive signals indicated at (d) and (g), respectively, in FIG.14. As a result, the modulator driving voltage signal applied to theoptical modulator 52 becomes a potential difference [indicated at(d)-(g) in FIG. 14] that is applied between the signal electrodes 52 c,52 d. This is the same as the waveform of the first embodiment shown inFIG. 8. Subsequent control of the operating point is the same as in thefirst embodiment.

In FIG. 14, “1”, “0” correspond to the logic of the input electricalsignal. Because of push-pull drive, the drive signal (g) takes on the“1” level at the instant the drive signal (d) assumes the “1” level, andtherefore the envelope EU indicated at (d)-(g) becomes as shown atd1-g1. Similarly, the drive signal (g) takes on the “0” level at theinstant the drive signal (d) assumes the “0” level, and therefore theenvelope EL indicated at (d)-(g) becomes as shown at d0-g0. In FIG. 14,(e) is a signal that is the inverse of (a).

FIG. 15 illustrates a modification of the optical modulation apparatusaccording to the second embodiment. Components identical with thoseshown in FIG. 13 are designated by like reference characters. In thesecond embodiment, the electrodes for entering the drive signals arecommon with the electrodes for entering the bias voltages. However,these electrodes are provided as separate electrodes for drive signalsand for the bias voltages in this modification. Providing theseelectrodes as separate electrodes makes it possible to eliminatecapacitances for interrupting the bias voltages.

The modification of FIG. 15 differs from the second embodiment in thefollowing aspects:

(1) The electrode 52 c is separated into electrodes 52c₁, 52c₂ for thedrive signal and bias voltage, respectively, and the electrode 52 d isseparated into electrodes 52d₁, 52d₂ for the drive signal and biasvoltage, respectively.

(2) The drive signals output by the drive circuits 61, 62 are input tothe signal electrodes 52c₁, 52d₁, respectively.

(3) The bias voltage Vb1 (=Vb) is input to the bias voltage electrode52c₂ of the optical modulator 52, and the bias voltage Vb2 (=0) is inputto the bias voltage electrode 52d₂.

(4) The capacitances C1 and C2 are deleted.

By mutually inverting the amplitude modulating signals SAM₁, SAM₂, whichare applied to the drive circuits 61, 62, respectively, in the mannershown at (c) and (f) in FIG. 16, the drive circuits 61 and 62 are madeto output drive signals indicated at (d) and (g), respectively, in FIG.16. As a result of separately providing electrodes for the drive signalsand bias voltages in the modification of FIG. 15, the modulator drivingvoltage signal applied to the optical modulator 52 takes on a valueobtained by adding the bias voltage Vb1 (=Vb) of the bias electrode 52c₂to the potential difference input between the two signal electrodes52c₁, 52d₁. The waveform of this modulator driving voltage signal is asindicated at (d)+(h)−(g) in FIG. 16. This is a waveform similar to thatof the second embodiment.

(c) Third embodiment

FIG. 17 is a diagram showing the construction of an optical modulationapparatus according to a third embodiment. Components identical withthose of the second embodiment shown in FIG. 13 are designated by likereference characters. In the second embodiment, the drive signals SD,SD′ are both amplitude modulated by low-frequency signals, whereby theON and OFF sides of the modulator driving voltage signal are modulatedby low-frequency signals having the same phase. In the third embodiment,only one of the drive signals SD, SD′ is amplitude modulated by alow-frequency signal, whereby only the ON side or OFF side of themodulator driving voltage signal is modulated by the low-frequencysignal.

The third embodiment shown in FIG. 17 differs from the second embodimentof FIG. 13 in that the low-frequency signal SLF of frequency f₀ is inputto the gain control terminal of the drive circuit 61 as the amplitudemodulating signal SAM₁; the gain of the drive circuit 62 is notcontrolled. If the amplitude modulating signal SAM₁ is input to the gaincontrol terminal of the drive circuit 61, the gain of this drive circuitchanges. As a result, the drive circuit 61 outputs a drive signal of thekind shown at (c) in FIG. 18. Since the gain of the other drive circuit62 remains constant, however, this drive circuit outputs a drive signalof the kind indicated at (e) in FIG. 18, the center of this signal beingthe bias voltage Vb2 (=0). As a result, the modulator driving voltagesignal applied to the optical modulator 52 develops a potentialdifference [indicated at (c)-(e) in FIG. 18] that is applied between thesignal electrodes 52 c, 52 d. This is the waveform shown in FIG. 3.Accordingly, from this point onward the operating point is controlled insuch a manner that the low-frequency signal component of frequency f₀contained in the optical signal output by the optical modulator 52 willbecome zero.

(d) Fourth embodiment

FIG. 19 is a diagram showing the construction of an optical modulationapparatus according to a fourth embodiment. Components identical withthose of the second embodiment shown in FIG. 13 are designated by likereference characters. In the second embodiment, the drive signals SD,SD′ are modulated respectively by the low-frequency signal SAM₁ and thelow-frequency signal SAM₂ obtained by inverting the signal SAM₁, wherebythe ON and OFF sides of the modulator driving voltage signal aremodulated by low-frequency signals having the same phase. In the fourthembodiment, the drive signals SD, SD′ are amplitude modulated by thelow-frequency signals SAM₁, SAM₂ of identical phase but differentamplitude, whereby the ON and OFF sides of the modulator driving voltagesignal are modulated by low-frequency signals of identical phase butdifferent amplitudes.

The fourth embodiment of FIG. 19 differs from the second embodiment ofFIG. 13 in the following respects:

(1) First and second amplitude modulating signal generators 55 a, 55 bconstituted by amplifiers having different gains are provided instead ofthe amplitude modulating signal generator 55′, and the low-frequencysignal SLF is input to each of these signal generators.

(2) The amplitude modulating signal SAM₁ output by the first amplitudemodulating signal generator 55 a is input to the gain control terminalof the drive circuit 61, and the amplitude modulating signal SAM₂ outputby the second amplitude modulating signal generator 55 b is input to thegain control terminal of the drive circuit 62.

By varying the amplitudes of the amplitude modulating signals SAM₁,SAM₂, which are applied to the drive circuits 61, 62, respectively, asshown at (c) and (f) in FIG. 20, the drive circuits 61, 62 output thedrive signals indicated at (d) and (g), respectively, in FIG. 20. As aresult, the modulator driving voltage signal applied to the opticalmodulator 52 develops a potential difference [indicated at (d)-(g) inFIG. 20] that is applied between the signal electrodes 52 c, 52 d. Thisis the waveform shown in FIG. 4. Accordingly, from this point onward theoperating point is controlled in such a manner that the low-frequencysignal component of frequency f₀ contained in the optical signal outputby the optical modulator 52 will become zero.

(e) Fifth embodiment

FIG. 21 is a diagram showing the construction of an optical modulationapparatus according to a fifth embodiment. Components identical withthose of the second embodiment shown in FIG. 13 are designated by likereference characters. In the fifth embodiment, the drive signals SD, SD′are modulated respectively by the low-frequency signal SAM₁ and thelow-frequency signal SAM₂ obtained by inverting the signal SAM₁, wherebythe ON and OFF sides of the modulator driving voltage signal aremodulated by low-frequency signals having the same phase. In the fifthembodiment, the drive signals SD, SD′ are amplitude modulated by theamplitude modulating signals SAM₁, SAM₂ having different frequencies,whereby the ON and OFF sides of the modulator driving voltage signal aremodulated by signals of different frequencies.

The fifth embodiment of FIG. 21 differs from the second embodiment ofFIG. 13 in the following respects:

(1) First and second low-frequency signal oscillators 54 a, 54 b forgenerating low-frequency signals of frequencies f₁ and f₂, respectively,are provided.

(2) A low-frequency signal SLF₁ of frequency f₁ is input to the gaincontrol terminal of the drive circuit 61 as the amplitude modulatingsignals SAM₁, and a low-frequency signal SLF₂ of frequency f₂ is inputto the gain control terminal of the drive circuit 62 as the amplitudemodulating signals SAM₂.

(3) First and second phase comparators 57c1, 57c2 are provided. Theinputs to the phase comparators 57c1, 57c2 are the low-frequency signalsSLF₁, SLF₂ output by the low-frequency signal oscillators 54 a, 54 b,respectively, as well as the electrical signal conforming to thephotodiode output signal. The first and second phase comparators 57c1,57c2 detect and output the low-frequency signal components offrequencies f₁, f₂, respectively, contained in the photodetector output.

(4) Low-pass filters 57d1, 57d2 for smoothing the signals output by thefirst and second phase comparators 57c1, 57c2 are provided.

(5) An averaging circuit 57 e is provided. This circuit calculates theaverage value of the low-frequency components of frequencies f₁, f₂,which are contained in the photodetector output signal, and inputs theaverage value to the bias supply circuit 58.

(6) The bias supply circuit 58 controls the bias voltage in such amanner that the average value becomes zero.

When the amplitude modulating signal SAM₁ of frequency f₁ is input tothe gain control terminal of the drive circuit 61, the gain of the drivecircuit 61 changes and the latter outputs a drive signal of the kindindicated at (d) in FIG. 22. When the amplitude modulating signal SAM₂of frequency f₂ is input to the gain control terminal of the drivecircuit 62, the gain of the drive circuit 62 changes and the latteroutputs a drive signal of the kind indicated at (g) in FIG. 22. As aresult, the modulator driving voltage signal applied to the opticalmodulator 52 becomes a potential difference [indicated at (d)-(g) inFIG. 22] that is applied between the signal electrodes 52 c, 52 d. Thisis the same as the waveform shown in FIG. 5. From this point onward,therefore, the operating point is controlled in such a manner that thelow-frequency signal components of frequencies f₁, f₂ contained in theoptical signal output by the optical modulator 52 will become zero. Forexample, if the bias point of optical modulator 52 shifts from theoptimum value, both signal components of the low-frequencies f₁, f₂appear in the optical signal and the phase of each signal gives adirection of control for changing the bias point to the optimumposition. Accordingly, the averaging circuit 57 e calculates the averagevalue of both signal components, and bias control is performed in such amanner that this average value will become zero. This makes it possibleto improve the precision of control.

(f) Sixth embodiment

FIG. 23 is a diagram showing the construction of an optical modulationapparatus according to a sixth embodiment. Components identical withthose of the second embodiment shown in FIG. 13 are designated by likereference characters. In the second embodiment, the drive signals SD,SD′ are modulated respectively by the low-frequency signal SAM₁ and thelow-frequency signal SAM₂ obtained by inverting the signal SAM₁, wherebythe ON and OFF sides of the modulator driving voltage signal aremodulated by low-frequency signals having the same phase. In the sixthembodiment, the drive signals SD, SD′ are modulated respectively by thelow-frequency signal SAM₁ and low-frequency inverted signal SAM₂ ofdifferent phases, whereby the ON and OFF sides of the modulator drivingvoltage signal are modulated by low-frequency signals of differentphases.

The sixth embodiment of FIG. 23 differs from the second embodiment ofFIG. 13 in the following respects:

(1) A first delay circuit 71 for delaying the low-frequency signal SLFof frequency f₀ by a prescribed time T is provided, and the delayedsignal output by the delay circuit 71 is input to the gain controlterminal of the drive circuit 62 as the amplitude modulating signalsSAM2.

(2) A second delay circuit 72 for delaying the low-frequency signal SLFby half the delay time of the first delay circuit 71 (i.e., by T/2) andinputting the delayed signal to the phase comparator 57 c is provided.

(3) The phase comparator 57 c senses the direction of a shift in thebias point of the optical modulator by comparing the phase of thelow-frequency signal output by the delay circuit 72, whose delay is T/2,and the phase of the low-frequency signal component in the opticalsignal.

When the amplitude modulating signal SAM₁, which is obtained byinverting the low-frequency signal SLF, is input to the gain controlterminal of the drive circuit 61, the gain of the drive circuit changes.As a result, the drive circuit 61 outputs a drive signal of the kindindicated at (c) in FIG. 24. When the amplitude modulating signal SAM₂,which is obtained by delaying the phase of the low-frequency signal SLFby T, is input to the gain control terminal of the drive circuit 62, thegain of the drive circuit 62 changes and the drive circuit 62 outputs adrive signal of the kind indicated at (g) in FIG. 24. As a result, themodulator driving voltage signal applied to the optical modulator 52becomes a potential difference [indicated at (d)-(g) in FIG. 24] that isapplied between the signal electrodes 52 c, 52 d. This is the same asthe waveform of the first embodiment shown in FIG. 6. Accordingly, fromthis point onward the operating point is controlled in such a mannerthat the low-frequency signal component of frequency f₀ contained in theoptical signal output by the optical modulator 52 will become zero.

For example, if the bias point of the optical modulator 52 drifts, alow-frequency signal component delayed by T/2, which is a phase delaythat conforms to the direction of the shift, appears in the opticalsignal. This makes it possible to sense the direction of the shift inthe bias point of the optical modulator by comparing the phase of thelow-frequency signal, which enters via the delay circuit 72 that appliesthe delay T/2, and the phase of the low-frequency signal contained inthe optical signal.

(g) Seventh embodiment

FIG. 25 is a diagram showing the construction of an optical modulationapparatus according to a seventh embodiment. Components identical withthose of the first embodiment shown in FIG. 7 are designated by likereference characters. The associated signal waveforms are identical withthose of the first embodiment shown in FIG. 8.

In the first embodiment, a frequency component identical with thefrequency f₀ generated by the low-frequency oscillator 54 is detectedfrom the optical signal to control the operating point. However, as willbe understood from FIGS. 2 to 6, when the operating point of the opticalmodulator is at the optimum value, a low-frequency signal componentwhose frequency is twice the frequency f₀ (i.e., 2·f₀) appears in theoptical signal and this signal component takes on a maximum value.According to the seventh embodiment, therefore, the low-frequency signalcomponent of frequency 2·f₀ contained in the optical signal is detectedand the operating point is controlled so as to maximize this signalcomponent.

As shown in FIG. 25, the arrangement of the first embodiment isadditionally provided with a frequency multiplier 73 for doubling thefrequency f₀ of the low-frequency signal SLF output by the low-frequencyoscillator 54. The low-frequency signal of frequency 2·f₀ output by thefrequency multiplier 73 and the electrical signal conforming to theoptical signal output by the optical modulator are input to the phasecomparator 57 c, which proceeds to detect the low-frequency of frequency2·f₀ in the optical signal by phase comparison. The bias supply circuit58 controls the bias voltage, which is applied to the signal electrode52 c of the optical modulator, so as to maximize the low-frequencysignal component.

(h) Eighth embodiment

According to the foregoing embodiments, the modulation apparatusgenerates the two mutually complimentary drive signals (push-pull drivesignals) SD, SD′ each having the amplitude Vπ between the light-emissionculmination A and the neighboring light-extinction culmination B of thevoltage-optical output characteristic of the optical modulator 52, andinputs these drive signals to the respective signal electrodes of theoptical modulator, which is of the type driven on both sides thereof,whereby a modulator driving voltage signal of 2·Vπ is applied to theoptical modulator. However, if the objective is to eliminate chirping ofthe optical modulated signal by push-pull modulation and reducetransmission waveform degradation, it is not always necessary to apply amodulator driving voltage signal of 2·Vπ to the optical modulator.According to an eighth embodiment, therefore, two complimentary drivesignals SP, SP′ each of amplitude Vπ/2 are generated and the signals SP,SP′ are input to the respective signal electrodes of the opticalmodulator, which is of the type driven on both sides thereof, whereby amodulator driving voltage signal of Vπ is applied to the opticalmodulator to generate an NRZ optical signal or an RZ optical signal.

FIG. 26 is a diagram showing the construction of an optical modulationapparatus according to an eighth embodiment, and FIG. 27 is a waveformdiagram of the associated signal waveforms. Components identical withthose of the first embodiment shown in FIG. 7 are designated by likereference characters. The eighth embodiment of FIG. 26 differs from thefirst embodiment of FIG. 7 in the following respects:

(1) A push-pull drive signal generator 74 for generating twocomplimentary drive signals SP, SP′ each of amplitude Vπ/2 is provided.

(2) The low-frequency superimposing unit 55 superimposes thelow-frequency signal of frequency f₀ upon the drive signal SP in such amanner that the phases of the envelopes EU, EL on the ON and OFF sides,respectively, of the modulator driving voltage signal will be displacedfrom each other by 180° [see (c) of FIG. 27].

(3) The amplitude of the modulator driving voltage signal is made Vπ andthe phases of the envelopes EU, EL on the ON and OFF sides,respectively, of the modulator driving voltage signal are displaced fromeach other by 180° [see (d)-(f) of FIG. 27].

The center of the output signal from the low-frequency superimposingunit 55 and the center of the output signal from the drive circuit 62are made to coincide with the bias voltages Vb1 and Vb2 (=0V),respectively, of the signal electrodes 52 c, 52 d, respectively. Theseoutput signal waveforms, therefore, are as shown at (d) and (f) of FIG.27. As a result, the modulator driving voltage signal applied to theoptical modulator 52 has the amplitude Vπ [indicated by (d)-(f) in FIG.27], which corresponds to the potential difference across bothelectrodes, as well as the envelopes EU, EL, on the ON and OFF sides,modulated by the low frequency f₀ and at the 180° phase difference.

If the operating point of the optical modulator 52 drifts from theoptimum value, a low-frequency signal component having a phaseconforming to the direction of drift is produced in the optical signaloutput by the optical modulator 52. From this point onward, therefore,the bias voltage Vb1 of the optical modulator is controlled in adirection that will cancel out this low-frequency component.

Thus, in accordance with the eighth embodiment, chirping of the opticalmodulated signal is made zero by push-pull modulation, degradation of atransmission waveform can be reduced and drift of the operating pointcan be compensated for by control of bias voltage.

According to the eighth embodiment, the operating point is controlledupon detecting, from the optical signal, a frequency component identicalwith the frequency f₀ generated by the low-frequency oscillator 54.However, it is also possible to adopt an arrangement similar to that ofthe seventh embodiment (see FIG. 25), in which the low-frequency signalcomponent of frequency 2·f₀ contained in the optical signal is detectedand the operating point is controlled in such a manner that this signalcomponent takes on the maximum value.

Further, as described above, the low-frequency signal of frequency f₀ issuperimposed upon the drive signal SP in such a manner that the phasesof the envelopes EU, EL on the ON and OFF sides, respectively, of themodulator driving voltage signal will be displaced from each other by180°. However, it is also possible to adopt the following arrangements:

(1) a low-frequency signal is superimposed upon the drive signal SP orSP′ in such a manner that only one envelope of the envelopes EU, EL onthe ON and OFF sides, respectively, of the modulator driving voltagesignal will vary, or

(2) a low-frequency signal is superimposed upon the drive signals SP orSP′ in such a manner that the amplitudes of the envelopes EU, EL on theON and OFF sides, respectively, of the modulator driving voltage signalwill differ, or

(3) a low-frequency signal is superimposed upon the drive signals SP orSP′ in such a manner that the frequencies of the envelopes EU, EL on theON and OFF sides, respectively, of the modulator driving voltage signalwill differ, or

(4) a low-frequency signal is superimposed upon the drive signals SP orSP′ in such a manner that the phases of the envelopes EU, EL on the ONand OFF sides, respectively, of the modulator driving voltage signalwill differ, and the operating point is controlled upon detecting afrequency component, which is identical with the frequency f₀, from theoptical signal. When the operating point is controlled by any of thesemethods, the arrangements of the second through sixth embodiments can beapplied.

(i) Ninth embodiment

The periodicity of the voltage-optical output characteristic of theoptical modulator is such that 2·Vπ is equal to one period of voltage.Accordingly, the optical modulation apparatus can be provided with afunction for changing over the range of drive in terms of thevoltage-optical output characteristic of the optical modulator.

For example, in a case where modulation is carried out by a drivingamplitude of Vπ in NRZ modulation, the optical modulator is providedwith a function for shifting the bias voltage by Vπ between VbA and VbB,as shown in FIG. 28A, and the range of drive voltages is made to changefrom A to B by this shifting of the bias voltage. This shifting of theoperating point can be applied directly to the optical modulationapparatus (see FIG. 26) of the eighth embodiment, which generates theNRZ, RZ signals using the optical modulator that is driven on bothsides.

Further, in a case where modulation, such as optical duobinarymodulation, is performed by a driving amplitude of 2·Vπ, the opticalmodulation apparatus is provided with a function for shifting the biasvoltage by 2·Vπ between VbA and VbB, as shown in FIG. 28B, and the rangeof drive voltages is made to change from A to B by this shifting of thebias voltage.

The above-described changeover of the operating point can be applied toa case where chirping is set to a direction advantageous fortransmission and to a case where it is necessary to select a range inwhich the voltage-optical output characteristic has the proper shape.The changeover can be realized by intentionally shifting the biasvoltage by a fixed amount in response to an externally appliedchangeover signal.

FIG. 29 is a diagram showing the construction of an optical modulationapparatus according to a ninth embodiment having an operating-pointshifting function. Components identical with those of the firstembodiment shown in FIG. 7 are designated by like reference characters.

An operating-point changeover circuit 81 shifts the bias voltage by afixed amount of voltage in response to an externally applied changeoversignal CS, thereby changing over the range of drive of thevoltage-optical output characteristic.

An operating-point reset circuit 82 forcibly resets the bias point tozero in response to an operating-point reset signal RS. It is necessaryto reset the bias point (1) when the system is initially put intooperation and (2) when bias-point drift becomes so large during systemoperation that the bias voltage of the modulator, which is controlled toachieve stability, exceeds an allowable range. In such cases theoperating-point reset circuit 82 forcibly resets the bias point to zeroby the operating-point reset signal RS, which enters from the outside.

As shown in FIG. 29, the operating-point changeover circuit 81 isconstituted by a fixed-voltage power supply (although a variable-voltagepower supply is possible) and a switching arrangement for switching thebias supply line, and the operating-point reset circuit 82 isconstituted by a point of ground potential (GND) and a switchingarrangement for switching the bias supply line. However, other methodsare acceptable as long as the same operating-point changeover functionand operating-point reset function can be implemented. Further, theoperating-point changeover signal CS or operating-point reset signal RSis entered as necessary and the changeover of the operating point by theoperating-point changeover circuit or reset of the operating point bythe operating-point reset circuit is performed in accordance with theentered signal.

It should be noted that the components of FIG. 29 for shifting theoperating point and for resetting the operating point can be applied tothe eighth embodiment of FIG. 26 as is.

(j) Position of photodetector

In each of the foregoing embodiments, the optical modulator isexternally provided with the optical branching unit 56 and photodiode 57a. However, the same function can be achieved by incorporating aphotodiode 57 a in the LiNbO₃ substrate 52 of the optical modulator 52and detecting the light intensity of the light emission produced withinthe optical modulator, as shown in FIG. 30A. (See ECOC'97 vol. 2, pp.167-170, Y. Kubota et al., “10 Gb/s Ti; LiNbO3 Mach-Zehnder modulatorwith Built-in Monitor Photodiode Chip”.)

Specifically, at extinction of the MZ-type optical modulator 52, lightenergy is not actually extinguished even though light waves, displacedin phase by 180°, that propagate through branched optical waveguides 52a, 52 b are combined. Rather, a combining of modes takes place owing tothe width of the optical waveguides, and radiated light ascribable tosurplus modes radiates to the exterior of the optical waveguides frominterference points. If viewed from directly above the substrate, theradiated light radiates along an extension line of a branched opticalwaveguide 52 a′, as shown in FIG. 30B. Accordingly, a hole HL is cutinto the substrate at a prescribed position along the extension line,the photodiode 57 a is imbedded within the hole HL and is electricallywired. If this arrangement is adopted, an optical branching unit andexternal photodiode can be dispensed with, thereby making it possible tosimplify structure.

(k) Arrangement for dealing with input light of any polarization

If the fiber routing between the light source and optical modulator isof great length or the arrangement is not one in which polarization isfixed, then it is necessary to adopt an arrangement in which the opticalmodulator modulates light of any polarization. FIG. 31 shows an exampleof the construction of an MZ-type optical modulator capable of dealingwith such a situation. Shown in FIG. 31 are two branched opticalwaveguides 52 a, 52 b within the optical modulator, electrodes 52 c, 52d to which electrical signals for modulating the optical signals in therespective optical waveguides are input, and half-wave plates 91, 92inserted in the middles of the respective optical waveguides. Thehalf-wave plates are obtained by cutting holes into the substrate at theoptical waveguides and filling the holes with a material exhibitingbirefringence. The width of the half-wave plates is decided in such amanner that the optical path difference between the polarization modescaused by birefringence will be half the signal wavelength.

The efficiency of phase modulation in the optical waveguides of theoptical modulator is better in the TM mode than in the TE mode. If lightof any polarization having mixed TE-mode and TM-mode components entersthe modulator, the TM-mode components undergo phase modulation along thefirst half of the optical waveguides 52 a, 52 b (in front of thehalf-wave plates), after which they are converted to TE-mode componentsby the half-wave plates 91, 92. The TE-mode components then undergophase modulation along the second half of the optical waveguides (inback of the half-wave plates). Conversely, the TE-mode components do notundergo phase modulation in front of the half-wave plates, are convertedto TM-mode components by the half-wave plates 91, 92 and then are phasemodulated in the second half of the optical waveguides.

Accordingly, by giving consideration to design, such as the electrodelength for obtaining the amount of phase modulation necessary in thefirst and second halves of the optical waveguides, modulation can beperformed even in cases where light of any polarization impinges uponthe modulator.

Thus, in accordance with the present invention as described above, whenan optical modulator is driven by an electrical drive signal, which hasan amplitude of 2·Vπ between two light-emission culminations or twolight extinction culminations of a voltage-optical outputcharacteristic, a low-frequency signal component can be detectedreliably from the optical signal output of the optical modulator by wayof a simple arrangement, and fluctuation of the voltage-optical outputcharacteristic of the optical modulator, namely drift of the operatingpoint, can be compensated for using this low-frequency component. Byapplying the operating-point control method of the present invention tooptical duobinary modulation, the effects of waveform dispersion can bereduced. Moreover, push-pull drive makes it possible to reduce chirping.

In accordance with the present invention, when an optical modulator isdriven by a driving signal having an amplitude between a light-emissionculmination and a neighboring light-extinction culmination of avoltage-optical output characteristic, two complimentary drive signalseach having an amplitude of Vπ/2 are generated and the optical modulatoris subjected to push-pull drive by these drive signals. As a result,chirping can be reduced. Moreover, low-frequency signal components canbe detected reliably from the optical signal output by the opticalmodulator, and it is possible to compensate for drift of the operatingpoint.

In accordance with the present invention, a range used in modulation canbe shifted in the voltage-optical output characteristic of an opticalmodulator. As a result, chirping can be set to a direction advantageousfor transmission, or a range having the proper shape can be selectedfrom the shape of the voltage-optical output characteristic, and themodulator can be driven using this range.

In accordance with the present invention, an operating point on thevoltage-optical output characteristic of an optical modulator can be setto a prescribed initial point. Accordingly, in the event that bias-pointdrift becomes so large at the start of system operation or during systemoperation that the bias voltage exceeds an allowable range, the biaspoint can be forcibly reset to zero and the system restarted.

In accordance with the present invention, an arrangement is adopted inwhich a photodiode is imbedded in the substrate of an optical modulator,light leaking from an optical waveguide is detected and low-frequencycomponents are extracted from the detected light. This makes it possibleto dispense with an optical branching unit, thereby simplifying overallstructure.

In accordance with the present invention, a half-wave plate is insertedin the middle of a branched optical waveguide on each side of themodulator. This makes it possible to modulate light of any polarization.

In accordance with the present invention, control is performed so as tomaximize a signal component of frequency 2·f₀ contained in the output ofa photodetector, thereby making it possible to compensate foroperating-point drift that accompanies fluctuation of thevoltage-optical output characteristic of the optical modulator.

The present invention is such that when an optical modulator configuredto be driven on both its sides is used in optical duobinary modulation,NRZ modulation or RZ modulation, a low-frequency signal component can bedetected reliably from the optical signal output of the opticalmodulator by way of a simple arrangement, and it is possible tocompensate for operating-point drift that accompanies fluctuation of thevoltage-optical output characteristic of the optical modulator.

As many apparently widely different embodiments of the present inventioncan be made without departing from the spirit and scope thereof, it isto be understood that the invention is not limited to the specificembodiments thereof except as defined in the appended claims.

What is claimed is:
 1. An optical modulation apparatus including anoptical modulator having a voltage-optical output characteristics inwhich optical output varies periodically with respect to a voltage valueof an electrical drive signal, and a drive signal generator forgenerating an electrical device signal which drives the opticalmodulator by an amplitude between two light emission culminations or twolight extinction culminations of the voltage-optical outputcharacteristic, said apparatus comprising: a low-frequency oscillatorgenerating a prescribed low-frequency signal; a low-frequencysuperimposing unit superimposing the prescribed low-frequency signal onthe drive signal by varying a center level of the drive signal by saidlow-frequency signal; a low-frequency signal detection unit detectingoperating-point drift of the optical modulator based upon thelow-frequency signal component contained in an optical signal outputfrom said optical modulator; and an operating-point control unitcontrolling the operating point of the optical modulator in dependenceupon the drift of the operating point of the optical modulator.
 2. Anoptical modulation apparatus including an optical modulator having avoltage-optical characteristic in which optical output variesperiodically with respect to a voltage value of an electrical drivesignal, and a drive signal generator for generating an electrical drivesignal which drives the optical modulator by an amplitude between twolight emission culminations or two light extinction culminations of thevoltage-optical output characteristic, said apparatus comprising: alow-frequency oscillator generating a prescribed low-frequency signal; alow-frequency superimposing unit superimposing the prescribedlow-frequency signal on the drive signal in such a manner that phases ofupper and lower envelopes of said drive signal coincide; a low-frequencysignal detection unit detecting operating-point drift of the opticalmodulator based upon the low-frequency signal component contained in anoptical signal output from said optical modulator; and anoperating-point control unit controlling the operating point of theoptical modulator in dependence upon the drift of the operating point ofthe optical modulator.
 3. An optical modulation apparatus including anoptical modulator having a voltage-optical output characteristic inwhich optical output varies periodically with respect to a voltage valueof an electrical drive signal, and a drive signal generator forgenerating an electrical drive signal which drives the optical modulatorby an amplitude between two light emission culminations or two lightextinction culminations of the voltage-optical output characteristic,said apparatus comprising: a low-frequency oscillator generating aprescribed low-frequency signal; a low-frequency superimposing unitsuperimposing the prescribed low-frequency signal on the drive signal insuch a manner that only an upper or lower envelope of said drive signalvaries; a low-frequency signal detection unit detecting operating-pointdrift of the optical modulator based upon the low-frequency signalcomponent contained in an optical signal output from said opticalmodulator; and an operating-point control unit controlling the operatingpoint of the optical modulator in dependence upon the drift of theoperating point of the optical modulator.
 4. An optical modulationapparatus including an optical modulator having a voltage-optical outputcharacteristics in which optical output varies periodically with respectto a voltage value of an electrical drive signal, and a drive signalgenerator for generating an electrical drive signal which drives theoptical modulator by an amplitude between two light emissionculminations or two light extinction culminations of the voltage-opticaloutput characteristic, said apparatus comprising: a low-frequencyoscillator generating a prescribed low-frequency signal; a low-frequencysuperimposing unit superimposing the prescribed low-frequency signal onthe drive signal in such a manner that amplitudes of upper and lowerenvelopes of said drive signal differ; a low-frequency signal detectionunit detecting operating-point drift of the optical modulator based uponthe low-frequency signal component contained in an optical signal outputfrom said optical modulator; and an operating-point control unitcontrolling the operating point of the optical modulator in dependenceupon the drift of the operating point of the optical modulator.
 5. Anoptical modulation apparatus including an optical modulator having avoltage-optical output characteristic in which optical output variesperiodically with respect to a voltage value of an electrical drivesignal, and a drive signal generator for generating an electrical drivesignal which drives the optical modulator by an amplitude between twolight emission culminations or two light extinction culminations of thevoltage-optical output characteristic, said apparatus comprising: alow-frequency oscillator generating a prescribed low-frequency signal; alow-frequency superimposing unit superimposing the prescribedlow-frequency signal on the drive signal in such a manner thatfrequencies of upper and lower envelopes of said drive signal differ; alow-frequency signal detection unit detecting operating-point drift ofthe optical modulator based upon the low-frequency signal componentcontained in an optical signal output from said optical modulator; andan operating-point control unit controlling the operating point of theoptical modulator in dependence upon the drift of the operating point ofthe optical modulator.
 6. An optical modulation apparatus including anoptical modulator having a voltage-optical output characteristic inwhich optimal output varies periodically with respect to a voltage valueof an electrical drive signal, and a drive signal generator forgenerating an electrical drive signal which drives the optical modulatorby an amplitude between two light emission culminations or two lightextinction culminations of the voltage-optical output characteristic,said apparatus comprising: a low-frequency oscillator generating aprescribed low-frequency signal; a low-frequency superimposing unitsuperimposing the prescribed low-frequency signal on the drive signal insuch a manner that phases of upper and lower envelopes of said drivesignal differ; a low-frequency signal detection unit detectingoperating-point drift of the optical modulator based upon thelow-frequency signal component contained in an optical signal outputfrom said optical modulator; and an operating-point control unitcontrolling the operating point of the optical modulator in dependenceupon the drift of the operating point of the optical modulator.
 7. Anoptical modulation apparatus including an optical modulator havingoptical waveguides that branch on a light input side and merge on alight output side, two signal electrodes for applying phase modulationto optical signals in the branched optical waveguides on both sides andtwo drive-signal input terminals for inputting complimentary drivesignals to respective ones of said signal electrodes, and possessing avoltage-optical output characteristic in which optical output variesperiodically with respect to a voltage value of an electrical drivesignal; and a drive signal generator for generating complimentary drivesignals having an amplitude between a light-emission culmination and aneighbouring light-extinction culmination of the voltage-optical outputcharacteristic of said optical modulator, said apparatus comprising: alow-frequency-oscillator for generating a prescribed low-frequencysignal; a low-frequency superimposing unit superimposing the prescribedlow-frequency signal on the drive signal in such a manner that phases ofupper and lower envelopes of said drive signal coincide; a low-frequencysignal detection unit detecting operating-point drift of the opticalmodulator based upon the low-frequency signal component contained in anoptical signal output from said optical modulator; and anoperating-point control unit controlling the operating point of theoptical modulator in dependence upon the drift of the operating point ofthe optical modulator.
 8. An optical modulation apparatus including anoptical modulator having optical waveguides that branch on a light inputside and merge on a light output side, two signal electrodes forapplying phase modulation to optical signals in the branched opticalwaveguides on both sides and two drive-signal input terminals forinputting complimentary drive signals to respective ones of said signalelectrodes, and possessing a voltage-optical output characteristic inwhich optical output varies periodically with respect to a voltage valueof an electrical drive signal; and a drive signal generator forgenerating complimentary drive signals having an amplitude between alight-emission culmination and a neighbouring light-extinctionculmination of the voltage-optical output characteristic of said opticalmodulator, said apparatus comprising: a low-frequency-oscillator forgenerating a prescribed low-frequency signal; a low-frequencysuperimposing unit superimposing the prescribed low-frequency signal onthe drive signal in such a manner that only an upper or lower envelopeof said drive signal varies; a low-frequency signal detecting unitdetecting operating-point drift of the optical modulator based upon thelow-frequency signal component contained in an optical signal outputfrom said optical modulator; and an operating-point control unitcontrolling the operating point of the optical modulator in dependenceupon the drift of the operating point of the optical modulator.
 9. Anoptical modulation apparatus including an optical modulator havingoptical waveguides that branch on a light input side and merge on alight output side, two signal electrodes for applying phase modulationto optical signals in the branched optical waveguides on both sides andtwo drive-signal input terminals for inputting complimentary drivesignals to respective ones of said signal electrodes, and possessing avoltage-optical output characteristic in which optical output variesperiodically with respect to a voltage value of an electrical drivesignal; and a drive signal generator for generating complimentary drivesignals having an amplitude between a light-emission culmination and aneighbouring light-extinction culmination of the voltage-optical outputcharacteristics of said optical modulator, said apparatus comprising: alow-frequency-oscillator for generating a prescribed low-frequencysignal; a low-frequency superimposing unit superimposing the prescribedlow-frequency signal on the drive signal in such a manner thatamplitudes of upper and lower envelopes of said drive signal differ; alow-frequency signal detection unit detecting operating-point drift ofthe optical modulator based upon the low-frequency signal componentcontained in an optical signal output from said optical modulator; andan operating-point control unit controlling the operating point of theoptical modulator in dependence upon the drift of the operating point ofthe optical modulator.
 10. An optical modulation apparatus including anoptical modulator having optical waveguides that branch on a light inputside and merge on a light output side, two signal electrodes forapplying phase modulation to optical signals in the branched opticalwaveguides on both sides and two drive-signal input terminals forinputting complimentary drive signals to respective ones of said signalelectrodes, and possessing a voltage-optical output characteristics inwhich optical output varies periodically with respect to a voltage valueof an electrical drive signal; and a drive signal generator forgenerating complimentary drivesignals having an amplitude between alight-emission culmination and a neighbouring light-extinctionculmination of the voltage-optical output characteristic of said opticalmodulator, said apparatus comprising: a low-frequency-oscillator forgenerating a prescribed low-frequency signal; a low-frequencysuperimposing unit superimposing the prescribed low-frequency signal onthe drive signal in such a manner that frequencies of upper and lowerenvelopes of said drive signal differ; a low-frequency signal detectionunit detecting operating-point drift of the optical modulator based uponthe low-frequency signal component contained in an optical signal outputfrom said optical modulator; and an operating-point control unitcontrolling the operating point of the optical modulator in dependenceupon the drift of the operating point of the optical modulator.
 11. Anoptical modulation apparatus including an optical modulator havingoptical waveguides that branch on a light input side and merge on alight output side, two signal electrodes for applying phase modulationto optical signals in the branched optical waveguides on both sides andtwo drive-signal input terminals for inputting complimentarydrive-signals to respective ones of said signal electrodes, andpossessing a voltage-optical output characteristic in which opticaloutput varies periodically with respect to a voltage value of anelectrical drive signal; and a drive signal generator for generatingcomplimentary drive signals having an amplitude between a light-emissionculmination and a neighbouring light-extinction culmination of thevoltage-optical output characteristic of said optical modulator, saidapparatus comprising: a low-frequency-oscillator for generating aprescribed low-frequency signal; a low-frequency superimposing unitsuperimposing the prescribed low-frequency signal on the drive signal insuch a manner that phases of upper and lower envelopes of said drivesignal differ; a low-frequency signal detection unit detectingoperating-point drift of the optical modulator based upon thelow-frequency signal component contained in an optical signal outputfrom said optical modulator; and an operating-point control unitcontrolling the operating point of the optical modulator in dependenceupon the drift of the operating point of the optical modulator.
 12. Amethod of controlling an optical modulator, which has a voltage-opticaloutput characteristic in which optical output varies periodically withrespect to a voltage value of an electrical drive signal, by theelectrical drive signal, which has an amplitude between twolight-emission culminations or two light extinction culminations of thevoltage-optical output characteristic, said method comprising:superimposing a prescribed low-frequency signal on the drive signal insuch a manner that phases of upper and lower envelopes of said drivesignal coincide; detecting operating-point drift of the opticalmodulator based upon the low-frequency signal component contained in anoptical signal output from said optical modulator; controlling theoperating point of the optical modulator in dependence upon the drift ofthe operating point of the optical modulator; using, as said opticalmodulator, an optical modulator which includes optical waveguides thatbranch on a light input side and merge on a light output side two signalelectrodes for applying phase modulation to optical signals in thebranched optical waveguides on both sides, and two drive signal inputterminals for inputting complimentary drive-signals to respective onesof said signal electrodes; and generating two complimentary drivesignals having an amplitude between a light-emission culmination and aneighbouring light-extinction culmination of the voltage-optical outputcharacteristics of the optical modulator, and inputting thecomplimentary drive signals to respective ones of said signalelectrodes.
 13. A method of controlling an optical modulator, which as avoltage-optical output characteristic in which optical output variesperiodically with respect to a voltage value of an electrical drivesignal, by the electrical drive signal, which has an amplitude betweentwo light-emission culminations or two light extinction culminations ofthe voltage-optical output characteristic, said method comprising:superimposing a prescribed low-frequency signal on the drive signal insuch a manner that only an upper or lower envelope of said drive signalvaries; detecting operating-point drift of the optical modulator basedupon the low-frequency signal component contained in an optical signaloutput from said optical modulator; controlling the operating point ofthe optical modulator in dependence upon the drift of the operatingpoint of the optical modulator; using, as said optical modulator, anoptical modulator which includes optical waveguides that branch on alight input side and merge on a light output side two signal electrodesfor applying phase modulation to optical signals in the branched opticalwaveguides on both sides, and two drive-signal input terminals forinputting complimentary drive signals to respective ones of said signalelectrodes; and generating two complimentary drive signals having anamplitude between a light-emission culmination and a neighbouringlight-extinction culmination of the voltage-optical outputcharacteristic of the optical modulator, and inputting the complimentarydrive signals to respective ones of said signal electrodes.
 14. A methodof controlling an optical modulator, which has a voltage-optical outputcharacteristics in which optical output varies periodically with respectto a voltage value of an electrical drive signal, by the electricaldrive signal, which has an amplitude between two light-emissionculminations or two light extinction culminations of the voltage-opticaloutput characteristics, said method comprising: superimposing aprescribed low-frequency signal on the drive signal in such a mannerthat amplitudes of upper and lower envelopes of said drive signaldiffer; detecting operating-point drift of the optical modulator basedupon the low-frequency signal component contained in an optical signaloutput from said optical modulator; controlling the operating point ofthe optical modulator in dependence upon the drift of the operatingpoint of the optical modulator; using, as said optical modulator, anoptical modulator which includes optical waveguides that branch on alight input side and merge on a light output side two signal electrodesfor applying phase modulation to optical signals in the branched opticalwaveguides on both sides, and two drive-signal input terminals forinputting complimentary drive signals to respective ones of said signalelectrodes; and generating two complimentary drive signals having anamplitude between a light-emission culmination and a neighbouringlight-extinction culmination of the voltage-optical outputcharacteristic of the optical modulator, and inputting the complimentarydrive signals to respective ones of said signal electrodes.
 15. A methodof controlling an optical modulator, which has a voltage-optical outputcharacteristic in which optical output varies periodically with respectto a voltage value of an electrical drive signal, by the electricaldrive signal, which has an amplitude between two light-emissionculminations or two light extinction culminations of the voltage-opticaloutput characteristics, said method comprising: superimposing aprescribed low-frequency signal on the drive signal in such a mannerthat frequencies of upper and lower envelopes of said drive signaldiffer; detecting operating-point drift of the optical modulator basedupon the low-frequency signal component contained in an optical signaloutput from said optical modulator; controlling the operating point ofthe optical modulator in dependence upon the drift of the operatingpoint of the optical modulator; using, as said optical modulator, anoptical modulator which includes optical waveguides that branch onalight input side and merge on a light output side two signal electrodesfor applying phase modulation to optical signals in the branched opticalwaveguides on both sides, and two drive-signal input terminals forinputting complimentary drive signals to respective ones of said signalelectrodes; and generating two complimentary drive signals having anamplitude between a light-emission culmination and a neighbouringlight-extinction culmination of the voltage-optical outputcharacteristic of the optical modulator, and inputting the complimentarydrive signals to respective ones of said signal electrodes.
 16. A methodof controlling an optical modulator, which has a voltage-optical outputcharacteristic in which optical output varies periodically with respectto a voltage value of an electrical drive signal, by the electricaldrive signal, which has an amplitude between two light-emissionculminations or two light extinction culminations of the voltage-opticaloutput characteristic, said method comprising: superimposing aprescribed low-frequency signal on the drive signal in such a mannerthat phases of upper and lower envelopes of said drive signal differ;detecting operating-point drift of the optical modulator based upon thelow-frequency signal component contained in an optical signal outputfrom said optical modulator; controlling the operating point of theoptical modulator in dependence upon the drift of the operating point ofthe optical modulator; using, as said optical modulator, an opticalmodulator which includes optical waveguides that branch on a light inputside and merge on a light output side two signal electrodes for applyingphase modulation to optical signals in the branched optical waveguideson both sides, and two drive-signal input terminals for inputtingcomplimentary drive signals to respective ones of said signalelectrodes; and generating two complimentary drive signals having anamplitude between a light-emission culmination and a neighbouringlight-extinction culmination of the voltage-optical outputcharacteristic of the optical modulator, and inputting the complimentarydrive signals to respective ones of said signal electrodes.